1. Technical Field
The present invention relates to hexagonal ferrite particles and their production process as well as an anisotropic ferrite magnet and its production process.
2. Background Technique
Currently available oxide permanent magnet materials, for the most part, are Sr ferrites based on an M type hexagonal or other systems or, possibly, Ba ferrites, and their sintered or bonded magnets are produced. In order to increase the residual magnetic flux density, Br, of the properties of a magnet, it is important to increase its density and make it anisotropic by magnetic field pressing. In order to enhance another magnet property, say, coercive force, Hc, it is important to reduce ferrite particles to 1 .mu.m or less in size, thereby forming single domain particles.
So far, sintered magnets of Ba or Sr ferrites have been produced in the following manners. That is to say, iron oxide is mixed with the carbonate of Ba or Sr, and the mixture is then calcined for the completion of its ferritic reaction. After that, the calcined product is pulverized, pressed or otherwise compacted in a magnetic field, and sintered. In order to increase the Hc of a magnet, it is required to provide for ferrite particles of 1 .mu.m or less for pressing or compacting them in a magnetic field in consideration of grain growth at the time of sintering. To this end, two ways are available, one in which particles of a few .mu.m or more are reduced to 1 .mu.m or less after calcination and the other in which ferrite particles are synthesized in such a way that they have already been reduced to 1 .mu.m or less before pulverization.
Pulverization or the compaction of powders in a magnetic field may be achieved in two ways, say, a dry way and a wet way using a solvent. The wet procedure makes it easier to reduce ferrite particles to 1 .mu.m or less when compared with the dry procedure, and is favorable for enhancing the performance of a magnet as well, because of its excellent degree of orientation during the compaction in a magnetic field. For such wet pulverization, water has conventionally been used as the solvent.
On the other hand, increasing the proportion of single-domain particles may be achieved by co-precipitation, hydrothermal synthesis, or conventional ways in which fine materials are mixed together with high accuracy and the mixture is then calcined at a relatively low temperature at which no particle growth occurs. These procedures make it possible to obtain fine ferrite particles lying in the range of 0.01 to 1 .mu.m, thus allowing these particles to have a very high iHc (a high of about 6 kOe).
It is considered necessary to use such fine particles so as to increase the performance of an oxide permanent magnet, but never until now is there any report about anisotropic magnets using them and oriented in a magnetic field. Nor are they practically used.
This reason is that when an anisotropic magnet is produced with such fine particles, its iHc is increased, but its Br degrades so that no effect is obtained on improving its magnetic characteristics. And the reason for this Br degradation believes chiefly in the deterioration of the degree of orientation during the compaction in a magnetic field.
One reason for the degradation of the degree of orientation is that as the sizes of ferrite particles become smaller than required, for instance, are of the order of 0.1 .mu.m or less and the magnitude of magnetization (.sigma.s) is reduced as well, the rotational torques of the particles in a magnetic field are diminished. Another leading reason is that as the coercive forces (bHc) of the particles are increased as will be described later, they are likely to agglomerate together magnetically.
So far, the iHc of an M type Ba ferrite powder of 0.1 to 0.3 .mu.m, for instance, has reached a high of 4650 Oe, as set forth in the examples disclosed in JP-B 62-53443, but the .sigma.s is as low as 44 emu/g. Indeed, those examples are silent about any anisotropic magnet made up of such powder. Likewise, JP-B 49-38917 refers to powders of 1 .mu.m or less having an iHc value as high as 4250 Oe, but makes a mention of rubber magnet production alone.